Resonant cavity light emitting device

ABSTRACT

A light emitting device includes a resonant cavity formed by a reflective metal layer and a distributed Bragg reflector. Light is extracted from the resonant cavity through the distributed Bragg reflector. A light emitting region sandwiched between a layer of first conductivity type and a layer of second conductivity type is disposed in the resonant cavity. In some embodiments, first and second contacts are formed on the same side of the resonant cavity, forming a flip chip or epitaxy up device.

BACKGROUND

1. Field of the Invention

The present invention relates to semiconductor light emitting devicesand, in particular, to resonant cavity light emitting devices.

2. Description of Related Art

Semiconductor light emitting devices such as light emitting diodes(LEDs) are among the most efficient light sources currently available.Material systems currently of interest in the manufacture of highbrightness LEDs capable of operation across the visible spectrum includegroup III–V semiconductors, particularly binary, ternary, and quaternaryalloys of gallium, aluminum, indium, and nitrogen, also referred to asIII-nitride materials; and binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and phosphorus, also referred to asIII-phosphide materials. Such devices typically have a light emitting oractive region sandwiched between a p-doped region and an n-doped region.Often III-nitride devices are epitaxially grown on sapphire, siliconcarbide, or III-nitride substrates and III-phosphide devices areepitaxially grown on gallium arsenide by metal organic chemical vapordeposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxialtechniques.

Semiconductor light emitting devices may be included in a variety ofapplications including displays such as flat panel displays, indicatorlights such as traffic lights, and optical communication applications.In many applications, such as displays, it is desirable to have lightemitted in a preferred direction. However, light from such semiconductordevices is typically emitted isotropically from the active region.

One method to improve the light emission characteristics of a device byproviding more directed, anisotropic emission is proposed in U.S. Pat.No. 5,226,053, which teaches forming an optical cavity of an LED withina resonant Fabry-Perot cavity. FIG. 9 illustrates a resonant cavity LED(RCLED) according to U.S. Pat. No. 5,226,053. RCLED 110 comprises abottom electrode 111, a substrate 112, a quarter-wave stack of aplurality of pairs of semiconductor layers forming a bottom distributedBragg reflector (DBR) mirror, 113, one layer of each pair having arefractive index different from the refractive index of the other layerof the pair; a bottom confining layer, 114; an active layer or region,115; a top confining layer, 116; a highly-doped contact layer, 117, anda top electrode, 118, having a centrally located aperture 119. The topmirror of the Fabry-Perot cavity is formed by an interface betweencontact layer 117 and air within aperture 119. Such a mirror has areflectivity of the order of 0.25 to 0.35. The light emission takesplace through the aperture. U.S. Pat. No. 5,226,053 teaches that the useof a Fabry-Perot resonant cavity formed by the DBR and the contactlayer/air interface results in spontaneous light emission from theactive region, which is restricted to the modes of the cavity.

SUMMARY

In accordance with embodiments of the invention, a light emitting deviceincludes a resonant cavity formed by a reflective metal layer and adistributed Bragg reflector. Light is extracted from the resonant cavitythrough the distributed Bragg reflector. A light emitting regionsandwiched between a layer of first conductivity type and a layer ofsecond conductivity type is disposed in the resonant cavity. In someembodiments, first and second contacts are formed on the same side ofthe resonant cavity, forming a flip chip or epitaxy up device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view of a flip chip RCLED according to anembodiment of the invention.

FIG. 1B illustrates a p-contact for the device illustrated in FIG. 1A.

FIG. 1C illustrates an n-contact for the device illustrated in FIG. 1A.

FIGS. 2A and 2B are a plan view and a cross sectional view of acontacting scheme for a large junction flip chip RCLED.

FIGS. 3A and 3B are a plan view and a cross sectional view of a contactscheme for a small junction flip chip RCLED.

FIG. 4 illustrates a method of forming a III-phosphide RCLED accordingto FIG. 1A.

FIG. 5 is a cross sectional view an epitaxy-up RCLED according to anembodiment of the invention.

FIG. 6 is a plan view of the device of FIG. 5.

FIG. 7 is a cross sectional view of a vertical RCLED on a surfacemountable substrate, according to an embodiment of the invention.

FIG. 8 is a plan view of the device of FIG. 7.

FIG. 9 illustrates a prior art RCLED.

DETAILED DESCRIPTION

In accordance with the invention, a light emitting device such as an LEDis formed with the active region located in a resonant cavity formed bytwo reflective surfaces, typically a reflective metal surface and adistributed Bragg reflector (DBR). The device may be a vertical device,where the p- and n-contacts are formed on opposite sides of the device,an epitaxy-up device, where the p- and n-contacts are formed on the sameside of the device and light is extracted through the contact side, or aflip chip device, where the p- and n-contacts are formed on the sameside of the device and light is extracted through the side of the deviceopposite the contact side. Many of the embodiments illustrated below areIII-phosphide devices, however some embodiments of the invention may befabricated in other materials systems, such as III-nitride. In addition,in the embodiments described below, the location of the p- and n-typeregions of the device may be reversed.

FIG. 1A is a cross sectional view of a flip chip RCLED. The device ofFIG. 1A includes an active region 6 sandwiched between a p-dopedcladding region 5 and an n-doped cladding region 7. The wavelength oflight emitted by the active region may be controlled by selecting thewidth and composition of the layers in active region 6, as is known inthe art. An example of a suitable active region includes 3 or 4 quantumwells separated by barrier layers. An n-contact layer 8 separatesn-contact 10 from the n-doped cladding region 7. A p-contact 9 is formedon a p-doped contact layer 3. A distributed Bragg reflector 4 separatesp-doped cladding region 5 and p-contact layer 3. Light is extracted fromthe device through a transparent window that includes an undoped GaPwindow layer 1 and a GaP layer 2. The table below gives examples of thethickness, composition, and dopant appropriate for each of layers 3, 5,6, 7, and 8.

P-doped contact layer 3 Five micron thick layer of Mg doped GaP P-dopedcladding region 5 One micron thick layer of Mg doped AlInP Quantum wellsof active 125 angstrom thick layers of undoped InGaP region 6 Barrierlayers of active 125 angstrom thick layers of undoped AlInP region 6N-doped cladding region 7 One micron thick layer of Te doped AlInPN-contact layer 8 500 angstrom thick layer of Te doped GaInP

The characteristics given below for each layer are examples and are notmeant to be limiting. More information on selecting the appropriatecharacteristics of the layers of the device may be found in chapters 1–3of Semiconductors and Semimetals, Volume 64, Electroluminescence I,Academic Press, San Francisco, 2000, Gerd Mueller, ed., which isincorporated herein by reference.

P-contact 9 and n-contact 10 may be multilayer structures, asillustrated in FIGS. 1B and 1C. FIG. 1B illustrates an example of amultilayer p-contact. A layer of Au—Zn alloy 9A is formed adjacent tocontact layer 3, in order to provide ohmic contact to the semiconductorlayer. Au—Zn layer 9A is protected by a guard metal layer 9B of, forexample, a sandwich of TiW, TiW:N, and TiW. A thick contact layer 9C,such as gold, is then formed over guard layer 9B. The ohmic layer 9A andguard layer 9B may cover all or just a portion of the semiconductorlayer under reflector 9C.

A multilayer n-contact may have a similar structure, as illustrated inFIG. 1C. A layer of Au—Ge alloy 10A is formed adjacent to contact layer8, in order to provide ohmic contact to the semiconductor layer. Au—Gelayer 10A is protected by a guard metal layer 10B of, for example, asandwich of TiW, TiW:N, and TiW. A thick reflective layer 10C of Au isdeposited over layers 10A and 10B. The ohmic layer 10A and guard layer10B may cover all or just a portion of the semiconductor layer underreflector 10C.

The resonant cavity is formed by DBR 4 and the mirror created byreflective n-contact 10. The length of the cavity is typically aninteger multiple of λ/2, where λ is the wavelength of light emitted bythe active region in the resonant cavity. Since the thickness of theactive region may be fixed by the wavelength of light desired, thelength of the resonant cavity may be adjusted by adjusting the thicknessof cladding regions 5 and 7. The resonant cavity must be long enough toprovide sufficient material to form a functioning device. Often, thecavity length is between about 10λ/2 and about 50λ/2. The physicalcenter or center of brightness of the active region is placed near anantinode relative to the DBR.

In the embodiment of FIG. 1A, the mirror and the DBR are selected to bereflective to light generated by the active region and conductive. Sincethe DBR illustrated in FIG. 1A is epitaxially grown with the devicelayers, it must also be lattice matched to the layers on either side ofthe DBR. Light is extracted from the device through DBR 4. A suitablereflectivity for DBR 4 is between about 60% and about 90%, preferablyabout 75% to about 85%. In the III-phosphide device described above, DBR4 may be comprised of, for example, alternating layers of Mg-doped AIInPand (Al_(x)Ga_(1−x))_(0.48)In_(0.52)P layers. The fraction of aluminum xin DBR 4 is typically more than the fraction of aluminum in a lightemitting layer of the active region. For example, the fraction ofaluminum in DBR 4 may be 5% greater than the fraction of aluminum in theactive region. The composition of the layers included in DBR 4 isselected such that the DBR is transparent to light from the activeregion. The thickness and number of layers included in DBR 4 may beselected to create a desired reflectivity, as is known in the art.Suitably reflective n-contact materials for III-phosphide andIII-nitride devices include Ag, Au, Al, Pt, Pd, Re, Ru, Rh, In, Cr, oralloys thereof. In some embodiments, the mirror material is selected tohave a reflectivity of at least 75%. In other embodiments, the mirror isat least 80% reflective, and preferably at least 90% reflective. Asillustrated in FIG. 1C, the entire surface of the mirror need not be thesame material, and therefore may not have the same reflectivity. Forexample, a gold reflector may be formed over one or more sections ofAu—Ge ohmic contacts.

FIGS. 2A and 2B illustrate an arrangement of contacts 9 and 10 for alarge junction device (that is, a device having an area greater thanabout 400×400 μm²) according to FIG. 1A. FIG. 2A is a plan view and FIG.2B is a cross section taken along line DD. Layers 20 are the same aslayers 1, 2, 3, 4, 5, 6, 7, and 8 of FIG. 1A. The active region of FIG.1A is divided into four isolated regions, in order to minimize thedistance between the p- and n-contacts. P-contact 9 surrounds andinterposes the four regions. N-contacts 9 are formed on the fourregions. P- and n-contacts 9 and 10 are electrically isolated from eachother by air or by optional insulating layer 22. Six p-submountconnections 23 and sixteen n-submount connections 24 are deposited onthe p- and n-contacts to form a surface suitable for connecting thedevice to a submount. The submount is often a silicon integrated circuitattached to the device by solder joints. In such embodiments, the p- andn-submount connections may be, for example, solderable metals. In otherembodiments, the device is connected to the submount by gold bonds, coldwelding, or thermocompression bonding.

FIGS. 3A and 3B illustrate an arrangement of contacts 9 and 10 for asmall junction device (that is, a device having an area less than about400×400 μm²) according to FIG. 1A. FIG. 3A is a plan view and FIG. 3B isa cross section taken along line CC. Layers 20 are the same as layers 1,2, 3, 4, 5, 6, 7, and 8 of FIG. 1A. The device shown in FIGS. 3A and 3Bhas a single via 21 etched down to a p-type layer of epitaxial structure20 below the active region. A p-contact 9 is deposited in via 21. P-via21 is located at the center of the device to provide uniform current andlight emission. An n-contact 10 provides electrical contact to then-side of the active region of epitaxial structure 20. N-contact 10 isseparated from the p-contact 9 by one or more dielectric layers 22. Twon-submount connections 24 connect to n-contact 10 and a p-submountconnection 23 connects to p-contact 9. P-submount connection 23 may belocated anywhere within p-contact region 9 (surrounded by insulatinglayer 22) and need not necessarily be located directly over via 21.Similarly, n-submount connections 24 may be located anywhere onn-contact 10. As a result, the connection of the device to a submount isnot limited by the shape or placement of p-contact 9 and n-contact 10.

FIG. 4 illustrates a method for forming the device of FIG. 1A. First, instep 41, the epitaxial layers 11 of the device are grown by, forexample, metal organic chemical vapor deposition on a GaAs substrate.Contact region 8 is grown first, then cladding region 7, active region6, cladding region 5, DBR 4, and contact region 3. After contact region3, a thick region of GaP 2 may be grown by, for example, vapor phaseepitaxy. GaP layer 2 provides mechanical support and current spreadingto the other layers of the device. The lattice constants of regions 8,7, 6, 5, and 4 are controlled to be the same as the lattice constant ofthe GaAs growth substrate. Contact region 3 and VPE layer 2 are usuallyGaP, which is lattice-mismatched to GaAs. The composition of contactlayer 3 and VPE layer 2 are selected to be transparent to light emittedby the active region. The structure may then be optionallychemo-mechanically polished to form a surface suitable for bonding.

Contact layer 3 is then thermo-mechanically bonded in step 42 to a hostsubstrate including an undoped GaP window layer 1. An optional bondinglayer may be disposed between VPE layer 2 and GaP window layer 1. Asuitable bonding layer is Ga_(0.9)In_(0.1)P grown on window layer 1 by,for example, MOVPE. GaP layers 1 and 2 have an index of refractionclosely matched to the DBR and the active region, to avoid waveguidingand thus enhance light extraction from the device. Wafer bonding isdescribed in more detail in U.S. Pat. No. 5,376,580, titled “WaferBonding Of Light Emitting Diode Layers” and incorporated herein byreference. The GaAs growth substrate is removed in step 43, exposingcontact layer 8. In step 44, contact vias are etched through theepitaxial layers in order to expose parts of contact layer 3 on whichp-contacts 9 will be formed. The contact vias may be etched according tothe contacting schemes illustrated in FIGS. 2A and 3A, for example.

N- and p-contacts are formed in steps 45 and 46. First, ohmic contactlayers 9A and 10A, then guard layers 9B and 10B (FIGS. 1B and 1C) areformed on the appropriate semiconductor layers and alloyed to thesemiconductor layers by, for example, a rapid thermal anneal or annealin a furnace. A thick reflective layer is then deposited over the deviceand patterned to form reflectors 9C and 10C. In step 47, submountconnections such as solderable metals may be formed on completed n- andp-contacts. The device may then be mounted on a submount and packaged.The chip is shaped with beveled sides, as illustrated in FIG. 1A, bydicing the chip with a beveled blade. Shaped devices are described inmore detail in U.S. Pat. No. 6,229,160, titled “Light Extraction From ASemiconductor Light-Emitting Device Via Chip Shaping” and incorporatedherein by reference.

FIGS. 5 and 6 illustrate an epitaxy-up embodiment of the presentinvention. In the device of FIGS. 5 and 6, light is extracted from thedevice through the top of the device, i.e. side of the device on whichcontacts 9 and 10 are formed. Contact layer 8, DBR 4, cladding region 7,active region 6, cladding region 5, and contact layer 3 may have thesame characteristics as described above in reference to FIG. 1A. Layers14 may be shaped as illustrated in FIG. 5. Host substrate 12 is bondedto contact layer 3 by a reflective metal layer 13, which may be, forexample, gold, silver, or aluminum. Since light is extracted from thetop of the device, the host substrate bonded to the bottom of the devicelayers need not be transparent. Accordingly, in the device of FIG. 5host substrate 12 may be, for example, Si, metal, or glass.

The resonant cavity is formed by DBR 4, located between n-type claddingregion 7 and n-contact layer 8, and reflective metal layer 13, locatedbetween host substrate 12 and contact layer 3. The structure andreflectivity of DBR 4 and reflective metal layer 13 may be the same asDBR 4 and contact 10 of FIG. 1A. As in FIG. 1A, DBR 4 is the surfacethrough which light is extracted from the resonant cavity. The length ofthe resonant cavity and the placement of active region 6 may be the sameas described above in reference to FIG. 1A.

FIG. 6 is a plan view of the device of FIG. 5. Since light is extractedfrom the resonant cavity through DBR 4 and from the device through thesurface of contact layer 8, n-contact 10 is constructed to cover aslittle of the surface of contact layer 8 as is required to form anelectrical contact to layer 8. Usually, contact 10 is depositing suchthat 90% of the contact layer 8 is open, and contact 10 contacts 10% ofcontact layer 8. In the embodiment illustrated in FIG. 6, n-contact 10forms a ring around contact layer 8. In other embodiments, n-contact 10may be a mesh. P-contact 9 is formed on contact layer 3 adjacent tolayers 14. The p- and n-contacts may be connected to bonding pads on asubmount by traces, or the through conductive regions in host substrate12, as illustrated in FIG. 7.

In some embodiments, the device illustrated in FIGS. 5 and 6 is formedas described above in FIG. 4, except that DBR 4 is grown between contactlayer 8 and cladding region 7, and host substrate 12 and reflectivelayer 13 replace transparent layers 1 and 2. The structure of FIG. 5offers an advantage over the structure of FIG. 1A in that DBR 4 is grownbefore the p-type layers of the device in the structure of FIG. 5.P-type material is known to grow with a rougher surface morphology thanother material, which may disrupt the mirror planarity and function of aDBR grown over the p-type material. In some embodiments, DBR 4 andcontact layer 8 are grown on a separate GaAs substrate, then waferbonded to cladding region 7. The GaAs substrate is then removed.

FIGS. 7 and 8 illustrate a vertical embodiment of the present invention.Like the device of FIG. 5, light is extracted from the device from thesurface of contact layer 8. The resonant cavity is formed by DBR 4 andreflective metal layer 13. N-contact 10, which forms a ring aroundcontact layer 8, electrically contacts the n-side of the device. Aportion of reflective metal layer 13 electrically contacts the p-side ofthe device. The host substrate bonded to contact layer 3 includesconductive regions 16 and 17 and insulating layer 12, which electricallyconnect to n-contact 10 and reflective p-contact 13. Host substrate 12may be, for example, a silicon substrate, and conductive regions 16 and17 may be metals or heavily doped semiconductor regions. Contact 10connects to conductive region 16 by a layer of contact materialextending down one or more sides of epitaxial layers 14. Layers 14 areelectrically isolated from the contact material by an insulating layer15. The portions of reflective metal layer 13 electrically connected top-contact conductive region 17 and n-contact conductive region 16 arealso isolated by an insulating layer 15. The device illustrated in FIGS.7 and 8 may be surface mounted on another device by, for example, solderjoints electrically and physically connected to tbe back side ofconductive regions 16 and 17. Examples of host substrates surfacemountable in this manner are described in more detail in U.S. Pat. No.6,876,008, granted Apr. 5, 2005 titled “Mount for Semiconductor LightEmitting Device,” and incorporated herein by this reference.

The inclusion of a resonant cavity in a light emitting device offers theadvantage of concentrating light generated by the active region in adirection normal to the surface of the device. When included inapplications requiring light to be emitted in a particular direction,such as displays, resonant cavity devices may be more efficient thandevices without resonant cavities that emit light isotropically, sincelight emitted in directions other than the preferred direction is likelyto be lost. In addition, since resonant cavity devices may beconstructed to concentrate light in a direction normal to the surface ofthe device, structures typically used to capture light emitted in otherdirections, such as reflector cups, may not be needed with resonantcavity devices, beneficially reducing the source size of a resonantcavity device.

Though the embodiments described above are III-phosphide devices, someembodiments may be applied to III-nitride devices. For example, in thedevice of FIG. 1A, in a III-nitride device, DBR 4 may be formed on theopposite side of contact layer 3 and may be, for example, dielectriclayers deposited after growth of the device. In the device of FIG. 5,DBR 4 may also be dielectric layers deposited after growth of thedevice. Since contact 10 cannot be deposited on dielectric layers,portions of DBR 4 may be removed to expose mesas of layer 7 on whichcontact 10 may be formed.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

1. A light emitting device comprising: a structure comprising: a lightemitting region disposed between a region of first conductivity type anda region of second conductivity type; and a distributed Bragg reflector;a substrate; a first contact electrically connected to the region offirst conductivity type; and a second contact electrically connected tothe region of second conductivity type, the second contact comprising afirst metal layer having a reflectivity to light emitted by the lightemitting region greater than 75%, the first metal layer being disposedbetween the substrate and the structure; wherein the first contact iselectrically connected to the substrate by a second metal layerextending along a side surface of the structure.
 2. The device of claim1 wherein the light emitting region is disposed between the distributedBragg reflector and the first metal layer.
 3. The device of claim 1wherein the first metal layer comprises a metal selected from the groupof Ag, Au, Al, Pt, Pd, Re, Ru, Rh, Tn, Cr, and alloys thereof.
 4. Thedevice of claim 1 wherein the distributed Bragg reflector has areflectivity to light emitted by the light emitting region between about60% and about 90%.
 5. The device of claim 1 wherein the distributedBragg reflector and the first metal layer form a resonant cavity, andlight generated by the light emitting region is extracted from theresonant cavity through the distributed Bragg reflector.
 6. The deviceof claim 1 wherein the distributed Bragg reflector and the first metallayer form a resonant cavity, and a distance between the first metallayer and the distributed Bragg reflector is an integer multiple of λ/2,where λ is the wavelength of light emitted by the light emitting regionin the resonant cavity.
 7. The device of claim 1 wherein the distributedBragg reflector is disposed between the first contact and the region offirst conductivity type.
 8. The device of claim 7 wherein the firstcontact comprises a ring.
 9. The device of claim 7 wherein the firstcontact comprises a mesh.
 10. The device of claim 1 wherein the firstmetal layer has a reflectivity to light emitted by the light emittingregion greater than 80%.
 11. The device of claim 1 wherein thedistributed Bragg reflector has a reflectivity to light emitted by thelight emitting region between about 75% and about 85%.